The present invention relates to semiconductor devices which have semiconductor elements suitable for resistor elements, semiconductor wiring materials, connector elements, MOS (metal oxide semiconductor)-type FET (field effect transistor) gate electrodes and the like.
In the past, wiring materials and electrode materials for semiconductor devices have often used polysilicon. If this polysilicon is used as an intrinsic semiconductor, then it has a large resistance and so impurities are introduced into it so as to lower the value of the resistance. If the same amounts of donors and acceptors are introduced, they mutually cancel each other and the characteristics become the same as an intrinsic semiconductor, the value of the resistance becomes higher and hence only donors or acceptors are introduced in a high concentration and either an n.sup.+ type or a p.sup.+ type of semiconductor is used. The reason for the high concentration of either these donors or acceptors is to make the resistance as low as possible and to avoid the formation of a depletion layer.
In addition, conventionally, since there arises a rectifying action through the creation of a pn junction at a joint portion where a p-type polysilicon material and an n-type polysilicon material are directly joined, a metal such as aluminum or silicide (Si alloy) has been used as the material for the connection material of an ohmic connection between the p-type polysilicon material and the n-type polysilicon material but when a metal is used in the manufacture or a semiconductor, there arises contamination of the manufacturing apparatus due to the metal and a different manufacturing apparatus has to be used before and after the process using the metal. Because of this, a space for wiring has to be provided inside the semiconductor device during its manufacture and so there is a problem that the wiring process becomes complex.
Furthermore, when an n-type or a p-type polysilicon is used as a wiring material or a resistor element, the resistivity will be too high if the impurity concentration is low, and although there will be a low resistance if the concentration is above a certain level, the value of the resistance hardly changes for a high concentration and so the resistance value must be controlled by changing the sectional area of the wiring and the resistor.
There is a work function as one of the characteristic parameters of the semiconductor but when polysilicon is used, this work function is determined by the type (such as donor or acceptor) and the impurity concentration. However, as described above, only an n.sup.+ type or a p.sup.+ type is used and so it is only possible to have two types of work function, and the difference between these work functions is approximately 1.1 V which is the same as the band gap of silicon. Here, the work function has the meaning of the energy difference .PHI. [eV] between the Fermier level of a solid, and the energy (vacuum level) of free space outside a solid, and is the energy necessary to free electrons in an absolute zero state from the solid and into the space.
When polysilicon is used as a semiconductor device, it is only possible to use these two types of work functions and so it is not possible to have free control of the work function.
In addition, the threshold voltage V.sub.TH of a MOS-type FET is largely influenced by the impurity concentration of the channel region and by the work function of the gate electrode but when an n.sup.+ type or a p.sup.+ type polysilicon is used as the gate electrode of a MOS-type FET, that work function has a width of approximately 1.1 V and that value is an extremely large value with respect to the threshold voltage V.sub.TH which is normally 0.7 V and since it is not possible to change the work function, the value of the threshold voltage V.sub.TH is controlled by changing the impurity concentration of the channel region of the MOS-type FET.
However, when the impurity concentration of the channel region is made a higher concentration, the operating speed becomes slower and so the impurity concentration of the channel region can only be changed within a certain range in practice, and there is the problem that the range of control of the threshold voltage V.sub.TH is limited.
Not only this, when polysilicon is used as a wring material or a resistor element, changing that resistivity is extremely troublesome since the wiring width (sectional area) has to be changed in order to have control to a required value.